switches MOSFET as a switch

  
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The transistor shown is a P-channel MOSFET acting as a `high-side switch`. More commonly, an N-channel MOSFET low-side switch is used, but what you have will work as long once you add something to the drain such as in this image of P-Channel MOSFET switch from : When the control goes `HI` the MOSFET switch is `OFF. ` When the control goes `LO` the
switches MOSFET as a switch - schematic

MOSFET acts as a switch, essentially shorting the drain and source. While this is not entirely true, it is a close approximation as long as the transistor is fully saturated. So the schematic you have shown can be used to switch 12V to something, but it will not connect the output to 0V unless a pull down resistor is used as shown in the above image. The opposite control scenario works for an N-channel MOSFET: LO control turns the switch off, HI control turns the switch ON. However, an N-channel is more suited to be a "LO-side switch" connecting the output to ground instead of VDD as in this image of an N-Channel MOSFET switch: The actual voltage level that determines if the FET is on or off is known as the gate threshold voltage. So called "logic level gates" work at lower voltages common in digital circuits such as 1. 8V, 3. 3V, or 5V. Although, crossing this threshold does not entirely turn the switch on or off, it merely allows the FET to start or stop conducting. The FET should be full saturated with the values noted in the datasheet to full turn ON or OFF. I should also add that it is pretty common practice to include a pull up resistor (10k or so) at the gate of the P-Channel MOSFET to keep it OFF in unknown states. Similarily, a pull down resistor is used at the gate of the N-Channel MOSFET to keep it OFF in unknown states.



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